Various types of communications systems are of such a character that proper framing is necessary to their operation. For example, in time-division multiplex transmission of independent digital bit streams, correct interpretation of the value of the incoming signal as one member of the transmitted symbol set, and direction of the interpreted symbol to the correct receiver, require proper framing. Likewise, in television transmission, picture elements displayed at the receiver must be in the same relative positions as those appearing at the transmitter, and, for a line-and-field scanned television signal (one in which the picture being transmitted is scanned in fields of adjacent parallel lines), proper framing is necessary to achieve this. Unless the data frames are especially short (for example, individual ASCII characters), correct synchronization is needed to maintain reliable communication during the periods between successive framing signals.
In ordinary (non-secure) communications systems, framing information often takes the form of a unique word, identification of which by the receiver establishes a particular instant as the datum from which time measurements may be made. The same word occurs periodically, at the same time in relation to each frame of data (for example, a television frame or field). Since the framing word occurs at a constant frequency and is identical for each frame, framing of the received intelligence is easily accomplished.
This technique is unsuitable, however, for a secure communications system, since an important security feature may be the denial of framing information. Framing denial can be accomplished by varying the time at which the intelligence occurs in relation to the framing word (scrambling); but some means must then be found of delivering this framing information to the intended recipient. If the addressee possesses the key to unscramble the data, no problem should occur. However, a further difficulty arises when sychronization information (such as a reference clock burst) is scrambled along with the intelligence. Such information is necessary to the framing process because it keeps the clock from drifting too far during the relatively long periods between framing pulses.